Mitt Romney has not done any campaigning the last few days. He's in Vermont with senior aides, preparing for debates next month. And even as President Obama prepares for tonight's big speech, campaign aides say he has been preparing for debates, too. NPR's Ari Shapiro asked past debate coaches what happens behind the scenes.

ARI SHAPIRO, BYLINE: Things used to be so much simpler, says Michael Waldman. He's a veteran of the Clinton administration.

MICHAEL WALDMAN: John F. Kennedy prepped for the first presidential televised debate by looking at a few note cards with one aide.

SHAPIRO: Nowadays, staffers assemble thick briefing books months in advance. Once the candidate has studied them, he stands on mock debate stage with a carefully chosen mock opponent and cameras rolling. That's so everyone can review the footage afterwards. Karen Hughes remembers long days in Crawford, Texas with George W. Bush and a parade of experts.

KAREN HUGHES: So it could be anything from Medicare to, if you had tax policy, you'd have the economists in the room. If it was foreign policy, you'd have the foreign policy specialists.

SHAPIRO: The goal is to exhaust every possible question and rehearse the perfect answer for each one, so by the time you get to the debate itself, there are no surprises. But memorizing the book alone is not enough. The candidates have to learn how to boil answers down and tie them into a campaign theme that makes sense to voters. Republican Ed Rollins coached presidential candidate Jack Kemp in 1988.

ED ROLLINS: You know, Jack had 40 ideas in his head at any one given moment. And you have to get the answers down to 30 or 60 seconds. It was kind of hard for him to do.

SHAPIRO: Once you've finally got the substance, style is just as critical. Al Gore learned that lesson the hard way. In the 2000 race, Gore had all the answers. Problem was, he acted like a guy who had all the answers. This sigh may have handed debate victory to George W. Bush.

(SOUNDBITE OF PRESIDENTIAL DEBATE)

PRESIDENT GEORGE W. BUSH: I've had a record of appointing judges in the state of Texas.

(SOUNDBITE OF SIGH)

That's what a governor gets to do. A governor...

SHAPIRO: After the debate, style turned to spin. The Bush team talked about how domineering Gore was, how he argued with the moderator and invaded Bush's space. It worked. Bush was judged the debate's winner. But four years later, Bush had body language problems of his own. Every time John Kerry answered a question, Bush would frown. Karen Hughes had to break the news to him after the debate.

HUGHES: He thought he did a good job in the debate. And I said, well, except for that cut shot when you kept scowling at the camera.

(LAUGHTER)

SHAPIRO: The candidates and their coaches know that one good zinger can win a debate, and they spend days searching for the perfect phrase. Ed Rollins worked with Ronald Reagan.

ROLLINS: If you hit on a couple of those, you sort of dominate the news media for the next couple of days.

SHAPIRO: For example, in 1984, people openly wondered whether Reagan was too old for another four years in office. This line was carefully scripted and powerfully effective.

(SOUNDBITE OF PRESIDENTIAL DEBATE)

PRESIDENT RONALD REAGAN: I am not going to exploit, for political purposes, my opponent's youth and inexperience.

SHAPIRO: One of the best-remembered zingers of all time came from Lloyd Bentsen in 1988. Debate coach Bob Shrum knew that Bentsen's opponent liked comparing himself to JFK. And sure enough, Dan Quayle made the comparison in the debate.

BOB SHRUM: And Lloyd Bentsen was entirely prepared to turn to him and say...

(SOUNDBITE OF VICE PRESIDENTIAL DEBATE)

LLOYD BENTSEN: Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy.

SHAPIRO: There's a balancing act, though. While debate coaches hate surprise, they don't want to over-prepare, either. A candidate with too much information in his head can seem unfocused. Karen Hughes says George W. Bush understood that danger as far back as the Texas governor's race.

HUGHES: He told the staff: If you have any ideas for what I should say, tell Karen. And it was a very smart move, because by debate day, my head was so crammed with stuff, I could not think.

SHAPIRO: All of this preparation comes down to one goal, and it isn't convincing a panel of judges, says former Clinton aide Michael Waldman.

WALDMAN: All these candidates know that it's not about winning points in a high school debating championship. It's about persuading the public one way or another that you're ready to be president.

SHAPIRO: It's a strange gauntlet to run, because there are lots of things a president has to be good at. But outside of a campaign, the president of the United States will never actually debate anyone.